The conventional two-piece metal can includes a can body, composed of a generally cylindrical side wall which is formed integrally with a bottom wall, and a separate end closure or lid is used to enclose the open end of the can body. One piece can bodies constructed from a relatively ductile metal, such as aluminum or steel, are normally fabricated by a drawing and ironing operation. In the drawing and ironing operation, a disc of metal is initially drawn into the form of a relatively shallow cup, and the walls of the cup are then ironed or thinned to the desired height.
If the bottom wall of the one-piece can body is flat, or normal to the axis of the cylindrical side wall, the internal pressure exerted by the product, such as beer or carbonated beverages, may cause the bottom wall to deform or bulge outwardly into a convex configuration, thereby making the can body unstable when stored in an upright condition. Increasing the thickness of the bottom wall will resist bulging of the bottom wall, but an increase in thickness of the bottom wall also results in an increase in material and shipping costs.
In an attempt to resist the internal pressure, the bottom walls of one-piece can bodies have been formed with an upwardly extending dome or cavity, and the dome acts to resist deformation under internal pressure. However, if the internal pressure is increased beyond a critical point, the concave domed area of the container may bulge out or invert. Under such a condition, the inverted bottom of the can body provides an unstable supporting surface and increases the tendency for the can body to tip when transported by high speed conveying lines or during storage and handling. In order to prevent the inversion of the concave dome in the can bottom, the bottom walls of one-piece can bodies have been fabricated with a substantially greater thickness than the side wall. For example, while the side wall of the can body may have a thickness of about 0.005 inch, the bottom wall will have a thickness in the range of 0.015 to 0.017 inch.
A further disadvantage of the concave, domed, bottom wall construction, is the difficulty of applying the protective coating to the interior of the can body. Due to the angle at which the concavity or dome is formed, it is difficult, using standard spraying equipment, to adequately coat the interior peripheral edge of the bottom wall.
As a concave bottom wall reduces the volume of the can body, the height or diameter of the can body must necessarily be increased to compensate for the concavity.
Rather than forming the bottom wall with a configuration to resist deformation under pressure, one-piece can bodies have also been formed with a bottom wall which is designed to be deformed outwardly under internal pressure, and yet maintain a stable supporting base when deformed. Can bodies of this type are described in U.S. Pat. No. 3,904,069. In the can body as disclosed in the aforementioned patent, the bottom wall includes a central concavity or upwardly extending depression which is bordered by an annular, relatively flat, panel section. When the can body is subjected to internal pressure, the panel section deforms or bulges outwardly, but the stability of the can body is retained, due to the fact that the bottom wall is deformed into a shape having a uniform circular ring upon which the can body rests.
U.S. Pat. No. 3,979,009 shows another type of can body which is designed to permit controlled bulging or flexing under internal pressure. In the can body of patent 3,979,009, the bottom wall is formed with a flat outer peripheral section, a central concave depression and an intermediate, annular dishedsection. When the can body is pressurized, the bottom wall will then be supported by the annular ring or joint between the central depression and the intermediate section.